The electrical resistance of the evaporation boats is generally determined by the amount of hexagonal boron nitride in the composition, as an electrically non-conductive component. The amount of boron nitride in the mixed material varies over a wide range and the mixed material can be adapted to meet many requirements. Furthermore, because of its properties, the addition of boron nitride makes the evaporation boats easier to shape, (cf. U.S. Pat. No. 3,181,968 and U.S. Pat. No. 3,673,118). Similarly, mixed materials that additionally contain aluminum nitride and/or silicon nitride, which are intended to improve the mechanical strength of the evaporation boats, are known (cf. U.S. Pat. Nos. 3,544,486 and 3,813,252 and GB Pat. No. 1,251,664).
To manufacture articles comprising the mixed materials, it is customary to subject homogeneous powder mixtures to hot pressing in graphite moulds. Temperatures of at least 1800.degree. C. are required to produce solid articles of adequate density. As is known, the boron nitride powder in the mixture, cannot be densified to a great extent alone without the concomitant use of sintering aids. The boron nitride also inhibits sintering of the powder mixtures. As a result, under the above-mentioned hot-pressing conditions, in general, only densities of less than 95% of the theoretical density are obtained when boron nitride is present in the powder mixture.
The addition of sintering aids makes it possible to obtain articles having a higher density by hot pressing mixed materials containing boron nitride. Examples of sintering aids are aluminum (cf. DE-AS No. 2,200,665 and U.S. Pat. No. 4,008,183), carbon (cf. U.S. Pat. No. 3,649,314), boric oxide (cf. U.S. Pat. No. 3,915,900) and nickel (cf. U.S. Pat. No. 4,268,314). Due to the use of sintering aids which collect preferentially at the grain boundaries during the sintering process, and can form glass-like phases, weak spots, which corrode easily during the vacuum evaporation of metals, are pre-programmed into the evaporation boats manufactured from these mixed materials. Such boats therefore have unsatisfactory corrosion behaviour in use.
Since hot pressing on an industrial scale is a cost-intensive process, the known mixed materials are manufactured, for economic reasons, in the form of large blocks from which the individual evaporation boats are cut and machined to the desired final dimensions. Depending on the size of the individual boats, up to one hundred boats can be manufactured from a hot-pressed block. In addition to the difficulty in attaining a high degree of densification, the manufacture of evaporation boats is made more difficult by the fact that a density gradient exists in the block. That is, the density distribution within the block is non-uniform and the properties of the block are direction-dependent. The non-uniform properties results in a resistance gradient over the length of each individual boat manufactured from the block. When the boat is used that is, when it is heated by the direct passage of current, the non-uniform resistance leads to non-uniform evaporation of the metal and spot overheating which causes corrosion at those points.
The density gradient in the block is caused by the biaxial die pressure which causes the formation of a double pressure cone. The double pressure cone causes non-uniform heat transfer which leads to differing degrees of densification. The maximum pressure is limited by the strength of the graphite of the press die so that elimination of the density gradient cannot be achieved by increasing the pressure.